The disclosure relates to turbofan engines. More particularly, the disclosure relates to main bearing arrangements for turbofan engines having epicyclic gear reductions.
Gas turbine engines and similar structures feature a number of subassemblies mounted for rotation relative to a fixed case structure. Such engines have a number of main bearings reacting radial and/or thrust loads. Examples of such bearings are rolling element bearings such as ball bearings and roller bearings. Typically such bearings all react radial loads. Some such bearings also react axial (thrust) loads (either unidirectionally or bidirectionally). Ball bearings typically react thrust loads bidirectionally. However, if the inner race is configured to engage just one longitudinal side of the balls while the outer race engages the other longitudinal side, the ball bearing will react thrust unidirectionally.
Tapered roller bearings typically react thrust unidirectionally. Two oppositely-directed tapered roller bearings may be paired or “duplexed” to react thrust bidirectionally. An example is found in the fan shaft bearings of U.S. Patent Application Publication 2011/0123326A1. Such fan shaft bearings are widely spaced to behave as two distinct bearings providing radial support at two spaced locations so as to adequately react overturning moments or torques normal to the bearing axis (e.g., pitch and yaw) and thus fully support the fan. Other duplexing examples involve closely spaced bearings which behave as a single bearing and their combination may be referred to as a single duplex bearing. Such a single duplex may need to have a longitudinally spaced apart additional bearing reacting radial loads (and optionally thrust loads) for their combination to react yaw and pitch loads.
US Patent Application Publications 2013/0025257A1 and 2013/0025258A1 disclose so-called three-spool engines wherein a high pressure spool comprises a high pressure compressor (HPC) and a high pressure turbine (HPT) respectively upstream of and downstream of a combustor. An intermediate spool comprises an intermediate pressure compressor (IPC) upstream of the HPC and an intermediate pressure turbine (IPT) downstream of the HPT. A low spool comprises a low pressure turbine (LPT) downstream of the IPT and driving the fan via a fan drive gear system. The exemplary low spool comprises only the LPT and associated shaft assembly and does not include any compressor stages.
Unless explicitly or implicitly indicated otherwise, the term “bearing” designates an entire bearing system (e.g., inner race, outer race and a circumferential array of rolling elements) rather than the individual rolling elements. The term “main bearing” designates a bearing used in a gas turbine engine to support the primary rotating structures within the engine that produce thrust. This is distinguished, for example, from an accessory bearing (which is a bearing that supports rotating structures that do not produce thrust such as the fuel pump or oil pump bearings in an accessory gearbox).